Confusion: \onehalfspacing vs. \spacing vs. Word vs. the world?

I have to write a paper with the requirement to use "1.5 line spacing" 12pt Times New Roman. Naturally I used

\usepackage{setspace}
\begin{document}
\onehalfspacing
Lorem ipsum…

So far so good. But I'm a litte paranoid so I checked it with Word and was suprised to get something completely different, so I made this comparison:

you may have to open that in a new tab.

But seriously? Am I missing something? Everyone recommends using \onehalfspacing but that can't be right? I checked the text with a bare minimum and the results are the same:

\documentclass[12pt]{article}

\usepackage[utf8]{inputenc}
\usepackage{mathptmx}
\usepackage{setspace}

\begin{document}
\onehalfspacing
Lorem ipsum...

I guess my question is: what setting should I use?

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The answers to What does 'double spacing' mean? might explain the unexpected result and provide solutions. –  Gonzalo Medina Aug 4 '12 at 21:19
As far as I understand it the main difference is the definition of "line spacing". (La)TeX seems the to define it as the baseline-to-baseline distance (see en.wikipedia.org/wiki/Leading). Word & co seem to use the total width of the inter-line white space as "line spacing". –  hakaze Aug 4 '12 at 21:22
You might want to check with whomever set the requirements. Some people are picky and assume their understanding of what 1.5 spacing means is the only correct one; other, more reasonable people will not be so dogmatic; and if the person who gave the requirement uses *TeX, then the setspace solution will be just fine. –  jon Aug 4 '12 at 23:48
Well, thanks. I'm studying law so I can be glad that people don't require me to use a typewriter. Since this is a "hard" requirement (they can give you a bad mark if you disregard it) I guess the \spaced{1.5} Version is the way to go... –  niclas197 Aug 5 '12 at 7:39